On the Movement of Animals, Part Two

Chap. I. Concerning the modes and mechanical operations by which the contraction of muscles can occur.

antagonists, and therefore, by equalized forces, the strengths of both muscles remain inert, or in rest, just as if they possessed no strength, like weights balanced on a scale; but from an equalized power, or one of no strength, the resistance of a vast weight of a thousand pounds cannot be lifted; therefore, the action of a vital muscle is not performed by the proper contractive force of fibrous machines. Furthermore, since such contractive force of muscles is meager and weak, it would be impossible for them to be able to exert that great force by which they suspend immense weights through vital motion.

This same thing is shown by another reason. Because the more muscles and their fibers are contracted and shortened, the more they become flaccid and the small machines from which they are composed are rendered lax. For we see that a machine exerts its maximum force when it is violently pulled, distracted, and elongated, like the strings of a tense lute; but when their length and interval is shortened, then they become flaccid and exert no force; therefore, the force of the muscle and its machines, being diminished after its contraction, will not be able to sustain huge weights.

From which it is deduced that the contractive force of the muscle fibers has the use of keeping the muscle tense without any corrugation—namely, in a disposition and aptitude so that it may be contracted most violently by a new external force at the command of the appetite.

Moreover, if the contraction, shortening, and inflation of a muscle were proper to itself—that is, were effected by the force of the machines from which the fibers are woven and not by an external force—whenever the distance between the ends of the same muscle is diminished as much as possible, as